The present invention relates to a toner used for developing electrostatic images in image forming methods, such as electrophotography and electrostatic printing, particularly a toner suited for heat-pressure fixation, and further a heat-fixing method using such a toner.
Hitherto, a large number of electrophotographic processes have been known, inclusive of those disclosed in U.S. Pat. Nos. 2,297,691; 3,666,363; and 4,071,361. In these processes, in general, an electrostatic latent image is formed on a photosensitive member comprising a photoconductive material by various means, then the latent image is developed with a toner, and the resultant toner image is, after being transferred onto a transfer material such as paper etc., via or without via an intermediate transfer member, as desired, fixed by heating, pressing, or heating and pressing, or with solvent vapor to obtain a copy or print carrying a fixed toner image.
As for the step of fixing the toner image onto a sheet (transfer) material such as paper which is the final step in the above-mentioned electrophotographic process, various methods and apparatus have been developed, of which the most popular one is a heating and pressing fixation system using hot rollers.
In the heating and pressing system using hot rollers, a transfer(-receiving) material, such as paper, carrying a toner image to be fixed is passed through hot rollers, while a surface of a hot roller having a releasability with the toner is caused to contact the toner image surface of the transfer material under pressure, to fix the toner image. In this method, as the hot roller surface and the toner image on the transfer material contact each other under a pressure, a very good heat efficiency is attained for melt-fixing the toner image onto the transfer material to afford quick fixation.
In the fixing step, however, a hot roller surface and a toner image contact each other in a softened or melted state and under a pressure, so that a part of the toner is transferred and attached to the fixing roller surface and then re-transferred to a subsequent transfer material to soil the transfer material. This is called an offset phenomenon. Accordingly, the prevention of a toner from being attached onto a hot fixing roller surface is considered as an important condition to be satisfied in the hot-roller fixing scheme.
Hitherto, for the purpose of preventing toner attachment onto a fixing roller surface, it has been practiced, for example, to form the roller surface of a material (e.g., silicone rubber or fluorine-containing resin) showing excellent releasability with respect to the toner and further coat the roller surface with a liquid showing good releasability, such as silicone oil, for the purpose of offset prevention and for preventing the fatigue of the roller surface material. However, while this is very effective form the viewpoint of toner offset prevention, this method is accompanied with the problem of resulting in a complicated fixing apparatus because of the necessity of a mechanism for supplying such an offset preventing liquid.
On the other hand, as transfer material for toner image fixation thereon, there have been generally used various grades of papers, coated papers and plastic films. Particularly, there is an increasing need for a transparency film for OHP (overhead projectors) (hereinafter called an xe2x80x9cOHP filmxe2x80x9d) as a means for presentation in various meetings or congress. Unlike papers, such an OHP film shows only a low oil absorptivity, so that in the case of using an offset-preventing oil, such as silicone liquid, a substantial amount oil is caused to remain on the OHP film after the fixation. This leads to difficulties, such as a lowering in transparence of the OHP film, thermal evaporation of the silicone oil and soiling therewith in the image forming apparatus, and also processing of the recovered oil.
As another method, it has been proposed and practiced to add waxes, such as low-molecular weight polyethylene or low-molecular weight polypropylene, which can be efficiently melted on heating, in order to provide an increased toner releasability.
The incorporation of waxes in toner particles have been proposed in JP-B 52-3304, JP-B 52-3305 and JP-A 57-52574.
The addition of waxes in toners has been also proposed in JP-A 3-50559, JP-A 2-79860, JP-A 1-109359, JP-A 62-14166, JP-A 61-273554, JP-A 61-94062, JP-A 61-138259, JP-A 60-252361, JP-A 60-252360 and JP-A 60-217366.
Such waxes are used for improving the anti-offset property of the toners at low temperatures and high temperatures and for increasing the toner fixability at low temperatures, but is on the other hand liable to cause difficulties of the toner, such as a lowering in storage stability, and a lowering in developing performance due to a temperature increase in the image forming machine and due to migration of the wax at the toner particle surfaces after a long period of standing. Further, the transparence of the OHP film image is also lowered by the wax addition. In view of these difficulties, as small an amount as possible of the wax addition is desirable.
For the above reason, various proposals have been made for improving the toner binder resins. For example, JP-B 51-23354 has proposed a toner comprising a vinyl copolymer having an appropriate degree of crosslinkage obtained through the use of a crosslinking agent and a molecular weight-adjusting agent. JP-B 55-6805 has proposed a toner comprising polymerized units of xcex1,xcex2-unsaturated ethylenic monomers and having a broadened molecular weight distribution as represented by a ratio of 35-40 between weight-average molecular weight and number-average molecular weight. Further, toners using a blend resin including a vinyl polymer and having specified Tg, molecular weight and gel content, have been proposed, as in publications described below.
It is true that a toner comprising a resin having a broader molecular weight distribution has a broader fixable temperature range between a fixing lower-limit temperature (or a lowest fixable temperature) and an offset temperature (or an offset initiation temperature) than a toner comprising a single resin having a narrower molecular weight distribution. However, such a toner having a broader molecular weight distribution is still accompanied with a difficulty that in a case where a sufficient offset-prevention effect is thought much of, it becomes difficult to achieve a sufficiently low fixing temperature, and on the other hand, in a case where the low-temperature fixability is thought much of, the offset prevention effect is liable to be insufficient.
For example, JP-A 56-158340 has proposed a toner including a binder resin comprising a low-molecular weight polymer and a high-molecular weight polymer. It is practically difficult for the binder resin to contain a crosslinked content, so that it becomes necessary to increase the molecular weight of the high-molecular weight polymer or increase the content of the high-molecular weight polymer in order to increase the anti-offset property at a high performance level. The compositional change in this direction tends to remarkably lower the resultant resin composition, so that it is difficult to attain practically satisfactory results.
As for a toner comprising a blend of a low-molecular weight polymer and a crosslinked polymer, JP-A 58-86558 has disclosed a toner comprising a low-molecular weight polymer and an insoluble and infusible polymer. The proposed toner may exhibit an improved fixability and an improved pulverizability of the resin composition. However, in view of a small Mw/Mn ratio of at most 3.5 between a weight-average molecular weight (Mw) and a number-average molecular weight (Mn) of the low-molecular weight polymer and a large content of 40-90 wt. % of the insoluble and infusible polymer, it is difficult to satisfy the anti-offset property of the toner and the pulverizability of the resin composition in combination at high performance levels. Practically, it is very difficult to produce a toner sufficiently satisfying the fixability and the anti-offset property without using a fixing device equipped with an offset-preventing liquid supply mechanism. Further, as the insoluble and infusible polymer is increased in amount, the melt viscosity in the melt-kneading step for toner production becomes very high, so that a much higher temperature than ordinary temperature is required for the melt-kneading, thus being liable to cause a thermal decomposition of the additives leading to a lowering in toner performances.
JP-A 56-16144 has proposed a toner having at least one peak in each of a molecular weight region of 103-8xc3x97104 and a molecular weight region of 105-2xc3x97106 according to a GPC molecular weight distribution. The toner may exhibit improved pulverizability of the binder resin, anti-offset property, prevention of filming or melt-sticking onto the photosensitive member and developing performance. Further improvements in anti-offset property and fixability are desired. It is difficult for the resin to comply with severer demands in these days while providing a further improved fixability and retaining or improving the other properties.
In this way, it is extremely difficult to realize the toner fixing performances, inclusive of low-temperature fixability and anti-offset property, at high performance levels.
JP-A 59-21845, JP-A 59-218460, JP-A 59-219755, JP-A 60-28665, JP-A 60-31147, JP-A 60-45259, JP-A 60-45260 and JP-A 3-197971 have proposed a toner having excellent fixing performances by containing specified amounts of insoluble matter in THF (tetrahydrofuran) or toluene. However, further improvements are desired for satisfying the low-temperature fixability and the continuous image forming performances in combination.
JP-A 60-31147 and JP-A 3-197971 have proposed toners further specifying the molecular weights of soluble matter. However, a further improvement in continue image forming performance is desired.
JP-A 3-251853 has proposed a toner obtained through suspension polymerization and exhibiting several peaks on a molecular weight distribution curve including a lowest molecular weight peak at 5xc3x97104 or below and a highest molecular weight peak at 2xc3x97105 or above. However, a further improvement is desired at present in respect of low-temperature fixability.
JP-A 10-63035 aims at an improved low-temperature fixability by using a binder resin containing a high-molecular weight component and a low-molecular weight component. However, due to a shearing force exerted during toner production, the high-molecular weight component is severed, so that the molecular weight control in the resin preparation stage is not reflected in toner performances, thus failing to satisfy the low-temperature fixability and anti-high-temperature offset property in combination. Further, from the viewpoint of toner viscoelasticity, a viscoelasticity effective for both low-temperature fixability and anti-high-temperature offset property cannot be attained by the molecular weight control of the resin alone.
JP-A 11-24310 also has proposed a toner containing a polyester resin having an Mw/Mn ratio of 10-1000 and also a Fischer-Tropsh wax added thereto. However, for similar reasons as described above, sufficiently good fixing performances cannot be attained.
A generic object of the present invention is to provide a toner having solved the above-mentioned problems.
A more specific object of the present invention is to provide a toner with excellent low-temperature fixability.
Another object of the present invention is to providing a toner which can be fixed under application of heat and pressure while applying only a minimum amount of oil or omitting the oil application at all.
Another object of the present invention is to provide a color toner capable of forming a high-quality full-color OHP film image excellent in transparence.
Another object of the present invention is to provide a toner with excellent environmental stability.
According to the present invention, there is provided a toner comprising: (i) a binder resin, (ii) a colorant, (iii) a hydrocarbon wax, (iv) a resin composition comprising at least a copolymer unit synthesized by reaction of a styrenic monomer with at least one monomer selected from the group consisting of nitrogen-containing vinyl monomers, carboxyl group-containing monomers, hydroxyl group-containing monomers, acrylate ester monomers and methacrylate ester monomers, and a hydrocarbon unit; and (v) an organometallic compound;
wherein the binder resin (i) comprises a polyester component in a proportion of at least 60 wt. % based on the binder resin (1), and
the toner has a molecular weight distribution as measured by gel permeation chromatography (GPC) including a weight-average molecular weight (Mw) of at least 4.0xc3x97104 and a ratio Mw/Mn of at least 50 between the weight-average molecular weight (Mw) and a number-average molecular weight (Mn).
According to another aspect of the present invention, there is provided a heat-fixing method, comprising: causing a fixing member to contact a toner image formed on a recording material, and imparting heat and pressure onto the toner image, thereby fixing the toner image onto the recording material, wherein
the toner image is fixed onto a fixing surface of the recording material under application of silicone oil supplied from the fixing member to the fixing surface at a ratio of 0-1xc3x9710xe2x88x927 g/cm2, and
the toner image is formed of the above-mentioned toner.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.